The ISDB-T (Integrated Services Digital Broadcasting—Terrestrial) BST-OFDM (band segmented transmission—orthogonal frequency division multiplexing) system is an example of what is often called a “single frequency network” (SFN).
In fact, the signal is generally made up of multiple carrier signals. Each carrier has a finite, though narrow, frequency range, and when we talk about the frequency of a single carrier signal, we mean the center frequency of the narrow range. For multi-carrier transmission, such as OFDM in an ISDB-T system, each carrier has a different respective center frequency, and the frequencies are orthogonal to each other. The multi-carrier signal is the sum of the signals of the multiple carriers and is a wideband signal which has a center frequency too. This center frequency is here referred to as the frequency of the signal.
In conventional broadcasting, a signal received by a relay station at one frequency is re-transmitted at another frequency in order to avoid interference. However, by contrast, in digital broadcasting, because of the scarceness of frequency resources, it is usually required that same signal is re-transmitted at the same frequency (i.e. over the same range of frequency) by all transmitters. This is what is meant by the term “single frequency network”.
FIG. 1 shows a block diagram of an example of the configuration of a known relay station (also referred to here as a repeater). To relay the terrestrial digital broadcast with SFN, waves 1 received from a master station by an antenna 3 are amplified by a relay broadcasting system 4 and re-transmitted by a second antenna 7 at the same frequency. The waves 1 include a master signal and a level of distortion.
The field strength of the transmitted wave is very intense compared with that of the received wave. Although the receiving antenna 3 is directed to the master station, the transmitting antenna 7 is arranged with a directivity which matches the shape of its service area, and the radio wave is normally radiated at a wider angle. Therefore, coupling waves 5 are received by the receiving antenna 3 at a level which increases with the directivity index of the transmitting antenna 7 to the direction of the receiving antenna 3, or conversely with the directivity index of the receiving antenna 3 to the direction of the transmitting antenna 7. Furthermore, the coupling waves 5 include components of the signal transmitted by the transmitting antenna 7 which reaches the receiving antenna 3 after being reflected by structures, trees and mountains near the relay station. Consequently, the coupling waves 5 are a synthetic signal having a varying delay time, intensity and phase.
A repeater including a cancellation algorithm has been proposed. As shown in FIG. 1, the amplification system 4 of the relay station includes not only an amplifier 13 but also a coupling waves cancellation unit 9 and a BPF (bandpass filter) 11. Note that the apparatus in FIG. 1 is simplified, and in practice often includes additional components such as D/A converter(s).
The coupling waves cancellation unit 9 is conventionally designed based on the assumption that the distortion of the incoming radio waves from the master station is small compared to the master wave itself. However, this assumption is not appropriate for a practical repeater transmission environment which generally has a multipath transmission channel (e.g. there are multiple radio paths between the master station and the relay station, each taking different times). In a multipath channel, the distortion or attenuation of the master wave can be serious at a particular frequency (i.e. over a particular frequency range), especially when the amplitudes of the paths arriving at different time are the same. This is called frequency selective fading. In the presence of deep frequency selective fading caused by such multipath interference, the conventional coupling cancellation unit cannot work well and diverges, because the conventional coupling cancellation unit algorithm does not take the effect of multipath interference into account. Furthermore, it is very difficult for a relay station to distinguish between multipath interference and coupling signals in the received signals. Moreover, for non-coherent modulation, the conventional PSK (phase shift keying) estimation method cannot work well if there is a multipath channel.
Both coupling interference and multipath interference vary with time. Thus, a further advantageous feature of a cancellation unit is that the hardware processing speed should be fast enough to update the adaptive filter of the coupling cancellation unit. The time taken depends on the amount of computation required to perform the updating and the hardware available.